Intrinsic-appearing gallium arsenide compound semiconductor material



Feb. 27, 1968 Filed June 22, 1965 oi 9' LL 5 smo v BY Mm R. E. JOHNSONET AL INTRINSIC-APPEARING GALLIUM ARSENIDE COMPOU SEMICONDUCTOR MATERIAL2 Sheets-Sheet 1 xlo K INVENTORS ROWLAND E. JOHNSON, EDWARD W. MEHAL,RALPH J. HACH, ROBERT W. HAISTY TTORNEYS' Feb. 27, 1968 R. E. JOHNSONETAL 3,371,051 INTRINSIC'APPEARING GALLIUM ARSENIDE COMPOUNDSEMICONDUCTOR MATERIAL Filed June 22, 1965 2 Sheets-Sheet 2 6\ POWERSOURCE GALLIUM ARSENIDE PHOTORESISTOR 34 2O 1 4 5 3 Q J M VOLTAGE f iAMPLIFIER AC ERROR SIGNAL TRANSISTOR CHOPPER DC ERROR L30 SIGNALINVENTORS ROWLAND E. JOHNSON, EDWARD w. MEHAL... -RALPH J. HACH, ByROBERT w. HAISTY mm,m m

ATTORNEYS United States Patent Ofiiiee 3,371,051 Patented Feb. 27, 19683,371,051 INTRINSIC-APPEARING GALLIUM ARSENIDE COMPOUND SEMICONDUCTORMATERIAL Rowland E. Johnson, 4323 Middleton 75229, and

Edward W. Mehal, 10505 Swallow 75218, both of Dallas, Tern; Ralph J.Hach, Ann Arbor, Mich.; and Robert W. Haisty, 31 Mohawk, Richardson,Tex. 75080 Continuation-impart of application Ser. No. 99,259. Thisapplication June 22, 1965, Ser. No. 465,915 2 Claims. (Cl. 252--518)This application is a continuation in part of our copending applicationSer. No. 99,259, filed Mar. 29, 1961, now abandoned.

This invention relates to gallium arsenide semiconductors, and moreparticularly, to intrinsic-appearing gallium arsenide material andmethods of making same.

Thermistors and photoresistors have been made from silicon and germaniumsemiconductor materials as well as from compressed and sintered cadmiumsulfide. The characteristic which is of necessity in photo diodes orconductors (sometimes referred to as photoresistors) and thermistcrs isthe ability to change resistivity responsive to changes in temperaturesor incident radiation. To obtain semiconductor material of suificientlyhigh resistivity at room temperatures to act as a changing impedanceunder the influence of temperature changes or light radiation changes,it is necessary for it to be high purity material, which in the case ofsilicon would contain not greater than impurity atoms per cubiccentimeter.

In the prior art it has been recognized that the high purity orrefinement of silicon or germanium resulted in intrinsic or highresistivity material which, as temperature increased, exhibited aresistivity decrease. In other words, temperature affords suificientactivation energy to excite the valence electrons into the conductionband thereby decreasing the resistivity of the material. Actually, theactivation energy necessary to excite these electrons into theconduction band is dependent on the width of the forbidden energy bandgap of the material because different activation energies are requiredfor different 'band gap materials. For silicon, the thermosensitive orphotosensitive range of changing resistivity ends above 300 C. Theforbidden energy band gap of silicon is 1.1 electron volts and asubstantial number of the electrons will be in the conduction band at300 C. thereby imparting low resistivity to the silicon.

One technique for making high purity, high resistivity germanium andsilicon is the well known process of float zoning. In this method amultiplicity of float zones are passed through, the material and theresistivity increases in gradual increments thereby becoming of higherand higher magnitude. To enhance the thermoand photosensitive propertiesof silicon, one patentee (Taft, US. Patent No. 2,860,219) suggestsintroducing gold in concentrations less than 10 atoms per cu. cm. toprovide higher sensitivity to the silicon with reference to resistivityin the range of temperature from minus 80 C. to plus 100 C. The photoconductive effect of the silicon impregnated with gold occurs in thetemperature range from -100 C. to 200 C.

The well known Group II-IV compound semiconductors have been exploitedby many for use in fabricating such devices as transistors, diodes,tunnel diodes, etc. The reason for explointing these materials, andparticularly, gallium arsenide is the fact that a greater latitude ofoperating characteristics can be achieved. For instance, galliumarsenide has a forbidden band gap of 1.35 electron volts. This wide bandgap makes it feasible to operate devices at several hundred degreescentigrade higher than either silicon or germanium. Likewise, mobilitiesof electron carriers are much greater for gallium arsenide than forsilicon or germanium. In accordance with the invention, devices such asthermosensitive and photosensitive resistors may be made which willoperate at temperatures up to 1,000 C. Heretofore, one of the majorproblems involved in making such a device was the impracticability ofobtaining high resistivity or intrinsic gallium arsenide. To beintrinsic, gallium arsenide should have total impurity carriers inconcentrations not greater than 10 to 10 per cu. cm. which is five orsix orders of magnitude lower than high purity silicon. Such purities ingallium arsenide are unknown.

In the present invention, the necessity for obtaining intrinsic orimpurity carrier concentrations in the range of 10' or 10 carriers percu. cm. in gallium arsenide is unnecessary. The invention avoids actualintrinsic gallium arsenide by providing a material which isintrinsicappearing but does not have low (10 to 10 carriers per cu. cm.)impurity concentrations. The gallium arsenide of this invention hastotal impurity concentration of 10 to 10 carriers per cmfi, but also hasenergy levels introduced therein at about 0.74 electron volt which isvery near the Fermi level of actual intrinsic gallium arsenide.

The procedure for obtaining the gallium arsenide material of theinvention is described by the following steps. First, the highest puritygallium and the highest purity arsenic obtainable are grown into acrystal of gallium arsenide. The crystal may be either extremely galliumrich or extremely arsenic rich, in other words, of nonstoichiometricproportions. Second, the gallium arsenide crystalline material is floatzoned by well known techniques which incrementally increases theresistivity. After a varying number of passes have been made, perhapsfive or six, the material suddenly changes from a resistivity range ofabout 1 ohm cm. to several meg ohmcentimeters. In other words, thegallium arsenide proceeds for a few passes in gradual incrementalamounts to increase in resistivity and then all or" a sudden itsresistivity changes 6 or 7 orders of magnitude. Such change iscompletely unobserved in the case of silicon and germanium and issomething totally unexpected.

Varying theories have been advanced to explain why the gallium arsenidebecomes intrinsic-appearing in resistivity when, in fact, the donor oracceptor impurity levels are 6 or 7 orders of magnitude higher thanwould be considered high purity gallium arsenide or truly intrinsicgallium arsenide.

In the process heretofore mentioned, gallium arsenide compoundsemiconductor material is obtained having an energy level existing atapproximately the center of the band gap. In other words, the galliumarsenide has an activation energy level of approximately 0.7 electronvolt. It is suggested that this middle of the forbidden band gap energylevel readily traps electrons from the conduction band therebyincreasing its resistivity. Thus, the material is intrinsic-appearingalthough it is not of the impurity concentration which is consideredhigh purity gallium arsenide to make it truly an intrinsic material.

Although the precise mechanism occurring in the case of gallium arsenideis unknown, it is theorized that one of three possible occurrencescreates the energy level of impurities that centers near the middle ofthe band gap. The first of these is that the gallium arsenide isnonstoichiometric having either an excess of arsenic or gallium. In thissituation it is believed for instance, that the arsenic enters a galliumsite in the crystal lattice structure having an energy level near themiddle of the band gap of the gallium arsenide. Thus, the arsensic wouldact as a trapping impurity and cause higher resistivity of the material.Second, the deep lying trap having an activation energy in the middle ofthe band gap could be caused by elements such as oxygen or ironpurposely doped into the gallium arsenide or merely present as anon-excludeble impurity during formation of the compound semiconductor.Third, another phenomenon which could cause gallium arsenide to becomeintrinsic-appearing is the presence of some impurity such as copper, forinstance, wherein the heat treating in the float zone process couldcause the copper to diifuse to donor impurity sites and pair with thedonor impurity thereby essentially neutralizing the electrical effectwith a consequent increase in resistivity.

The three theories heretofore mentioned are presented as plausibleexplanations of why the invention creates high resisitivity galliumarsenide which is intrinsic-appearing yet does not have sufficiently lowimpurity concentrations to be considered truly intrinsic galliumarsenide.

Quite surprisingly it was discovered that float zoning removes to alower concentration donor or acceptor impurities leaving trapping levelsat activation energies of about half the forbidden band gap of galliumarsenide. Thus, the dominating impurities affecting the resistivity ofthe gallium arsenide are at energy levels of trapping impurities, andcause the material to be intrinsic-appearing, high resistivity. Althoughdonor or acceptor impurity levels are in the gallium arsenide inquantities which would shift the Fermi level above or below the centerof the forbidden band gap, the Fermi level of the intrinsicappearinggallium arsenide remains near the center of the forbidden band gap.

Infrequently, crystals of gallium arsenide, prior to float zoning, willhave a high resistivity in the range of 40 to 80 megohm-cm. which couldwell indicate and support the theory of non-stoichiometry causing highresistivity. Normally, the gallium arsenide is not of sufficiently highresistivity to be useful as thermo-sensitive or photo-sensitive devicessince the energy level is not as large as 0.74 e.v. and the carrierlifetime is too short for good photoconductors. Therefore it is usuallynecessary to float zone the material to obtain sufliciently highresistivity.

In view of the foregoing, it is an object of the present invention toprovide an intrinsic-appearing gallium arsenide.

It is another object of the present invention to provide a method ofproducing gallium arsenide having a high resistivity with the Fermilevel at the center of the band p- It is another object of the presentinvention to provide a gallium arsenide material of high resistivitydoped with impurities having activation energies approximately half ofthe forbidden band gap energy.

It is another object of the invention to provide a gallium arsenidematerial having a resistivity of about 200 megohm-cm. at roomtemperature and capable of changing resistivity to kilohm-cm. at atemperature of about 200 C.

It is another object of the invention to form a gallium arsenidematerial having an energy level with an activation energy approximately0.7 electron volt.

Other objects and advantages of the invention will be readily apparentas the following detailed description becomes better understood inconjunction with the accompanying drawings wherein:

FIGURE 1 illustrates the change in resistance with temperature change ofthe intrinsic-appearing gallium arsenide material of the presentinvention having eight different temperature excursions plotted thereon;

FIGURE 2 illustrates the change in resistance of a device made fromintrinsic-appearing gallium arsenide material with respect to change inabsolute temperature after 10 cycles of various temperature excursions;

FIGURE 3 schematically illustrates a constant current control devicewith a gallium arsenide intrinsic-appearing bar as a photoresistor.

Although any known technique may be used for forming suitable galliumarsenide to make the intrinsic-appearing gallium arsenide of the presentinvention, a specific example of a method of making the gallium arsenideto be float zoned will now be presented.

Example I About 250 grams of gallium having a purity of 99.999 percentgallium was placed in a clean graphite boat. In another clean graphiteboat was placed 275 grams of arsenic having a purity of 99.999 percent.Both of the graphite boats were heated to 1,000 C. for about 15 minutesprior to placing gallium and arsenic therein. This operation served toclean the graphite boats of impurities. The graphite boat containinggallium was placed at one end of an ampule or bomb tube and the boatcontaining arsenic at the other end so that each end of the ampule ortube could be maintained at a diflerent temperature. The arsenic locatedin the ampule was heat treated at 350 C. The ampule or tube was thenevacuated and sealed. It should be appreciated that the arsenic could beplaced in the bomb tube directly and not in a carbon boat. The sectionof the tube wherein the gallium was located was heated to 1,240" C. andthe arsenic area of the tube was heated to 600 C. and maintained atthese respective temperatures for approximately 5 hours so that thecompound semiconductor gallium arsenide could form. The gallium arsenidewas allowed to freeze from one end to the other at a rate of about 1inch per hour. The first frozen end was cut off and sized to about .3 x.3 x 5 /2 inches for later float zoning.

The gallium arsenide bar cut to the dimensions above was etched with asolution of 1 part HCl to 2 parts nitric acid diluted 5050 with water.The bar was rinsed and air-dried at C. for about 30 minutes. This barwas then placed in a tube with excess arsenic, and the tube was thenevacuated and sealed. A molten zone was established at the top of thegallium arsenide bar and the arsenic vapor pressure within the tube wassupplied and controlled by maintaining an arsenic boiler at 575 C. Fivemolten passes were made through the sample of gallium arsenide afterwhich time a gallium arsenide single crystal was mounted on top of thesample and six more zone passes were made down through the sample toobtain a single crystal of gallium arsenide.

A resistivity measuring sample was cut from the top portion of the floatzoned crystal about .23 x .38 x .12 cm. Resistivity measurements weremade at various temperatures from 77 K. to 703 K. The resistivity rangedfrom a high at 77 K. of 12.9 10 to a low at 703 K. of 1.34 10 ohmcm.

Table I below contains data for resistivity at various temperaturesrecorded on the gallium arsenide compound prepared above.

TABLE I Temperature Resistivity,

ohm-cm. C. K.

23 296 39.8)(10 430 703 134x10 425 698 1.61 l0 420 693 1.07X10 410 6838.0)(10 400 673 5.35X10 390 663 2.67X10 370 643 295x10 360 633 322x10350 623 4.28X10 340 613 5.1)(10 330 603 59BX10 320 593 7.12X10 310 5837.7)(10 300 573 8.4)(10 280 553 9.88X10 260 533 2.11X10 240 513 308x10220 493 6.92X10 200 473 1.25X10 180 453 2.08X10 433 2.63X10 140 4133.75Xl0 120 393 415x10 The resistivity measurements were made by the twopoint probe method wherein contacts were placed on the surface of thewafer or bar at a given spacing for which the length to cross-sectionalarea ratio is determined. In this method current is passed through thebar and the voltage drop between the probes is determined from whichresistivity can be obtained by multiplying the crosssectional area todistance between probe ratio by the voltage divided by the current.

Example II Another bar of gallium arsenide was prepared by techniquessimilar to the ones employed in Example I, and the gallium arsenideExample II resistivity with temperature data is contained in Table II.

Example III Example III, another gallium arsenide temperature dependentelement was made in a manner similar to those made in Examples 1 and IIabove. This element was subjected to repeated temperature cycles todetermine the reproducibility of the resistivity at a specifictemperature. The results for 8 cycles are contained in Table III whereasthe resistivity versus temperature measured after 10 cycles is containedin Table IV. Table III and IV contain columns where the value is areciprocal of temperature 10 and conductivityX 10 TABLE III Resistivity,Temperature Conductivity, Run No ohm-em. mhos 10 1 5X10 518 1. 93 2x102X10" 526 1. 90 5X10 2 9X10 771 1.30 1.1 10 1 X10 759 1. 32 l X10 3 2X10444 2. 25 5x10 5x10 559 1. 79 2X10 5 1 10 628 1. 59 1X10 3X10 557 1.802X10 8 1. 5X10 736 i. 36 6. 7x10 1X10 770 1. 30 1X10 TABLE IVResistivity, Temperature Conductivity, Cycle ohm-em. mh0s 10 K. 1O K.

1. 5X10 445 2. 25 6. 7X10 1X10 455 2. 20 1X1!) 5X10 472 2.12 2X10 4X10485 2. 06 2. 5X10 3x10 489 2. O4 3. 3X10 1. 5X10 509 1. 96 6. 7X10 1x10524 1. 91 1x10 5X10 549 1.82 2x10 3X10 563 1.78 3. 3x10 2 2X10 578 1. 735x10 1. 5X10 587 1. 70 6. 7X10 1X10 604 1. 66 1X10 5X10 640 1. 56 2x103X10 671 1.49 3. 3X10 2X10 692 1. 45 5X10 1.5 10 727 1.38 6 7X10 1X10765 1. 31 1X10 7X10 789 1. 27 1. 4X10 6 10 807 1. 24 1 6X10 5 5X10817 1. 22 1.8)(10 5 5X10 795 1.26 1.8)(10 6. 0X10 781 1. 28 1. 6X10 7X10762 1. 31 1. 4X10 1. 5X10 699 1. 43 6. 7X10 2X10 676 1. 48 5X10 3X10645 1. 55 3. 3X10 5X10 623 1. 61 2X1O 1X10 590 1. 7(] 1X1() 1 5X10573 1. 75 6. 7 1O 2X10 563 1. 78. 5X10 3X10 548 1.82 3.3)(10 5X10 536 1.87' 2x10 1x10 512 1. 1x10 1 5X10 502 1.99 6.7)(10 2X10 492 2.03 5X103X10 482 2. 07 3. 3X10 5X10 455 2.15 2X10 1X10 447 2.24 1X10 Toillustrate the linearity of the thermo-sensitive gallium arsenideelements a factor of reciprocal of absolute tem perature 10 is plottedas an abscissa and the log of conductivity 10 is plotted as ordinate.FIGURES 1 and 2 illustrate the linearity of the thermistor through 8temperature cycles and 10 temperature. cycles, respectively.

Example IV TABLE V Temperature Resitivity, Free Electrons,

ohms-em. carriers/cc. r05 9. 03 1o 5. 54x16 152 8. 04 10 6. 24X10 200 1.06X10 4. 85x10" The activation energy of the trapping level. was about0.74 e.v. for Example IV.

FIGURE 3 illustrates the gallium arsenide element utilized as aphotoresistor in an apparatus for maintaining a constant current througha load resistance. The gallium arsenide photoresistor 1 is located inseries with a load resistance 2 varying from a nominal amount to 200megohms and a resistor 3. Photoresistor 1 is further coupled to anadjustable current source consisting of 6 /2 volt battery 4 with a Kpotentiometer 5 across it, and a 100K resistor 3 in series with thepotentiometer output. The other side of the gallium arsenidephotoresistor 1 is coupled to a power source 6. A transistor emitterfollower 20 has the base lead 21 connected between the resistor 3 andthe load resistance 2, the collector connected to a 6- volt DC supplyand the emitter grounded through resistor 24. The output of transistor20 is taken from the emitter resistor 24 and coupled into a transistorchopper 30. The

output of the transistor chopper is an AC error signal 33 which issuitably amplified by voltage amplifier 34, and the output of thevoltage amplifier 34 is coupled into a power amplifier 35 which is usedto drive lamp 40. In operation the load current is balanced against aset current provided by the current source comprised of battery 4,potentiometer 5, and resistor 3. If the load resistance 2 changescausing an unbalance current, the base 21 of the transistor emitterfollower 20 follows the unbalance current creating an error voltageacross the emitter follower resistor 24 developing a DC error signalwhich is coupled to the transistor chopper 30 to increase or decreasethe AC error signal 33. This AC error signal is amplified by voltageamplifier 34 and power amplifier 35 and thereby increases or decreasesthe intensity of the light 40 which is focused on the photoresistor 1.Increasing light intensity on the photoresistor 1 causes it to undergo adecrease in resistance and decreasing light intensity causes it toincrease the resistance of photoresistor 1. In this manner the totalresistance of photoresistor 1 and the load resistor 2 is maintained at aconstant amount.

As an example of the light sensitivity of gallium arsenide material, thegallium arsenide thermistor in Example I was utilized as thephotoresistor in the heretofore described circuit. In order to obtainwide variations in load resistance a second gallium arsenide thermistorunit was used which was photosensitive. This unit was capable of varyingin resistance from 140 megohms with room light up to 0.36 rnegohm underlight from a microscope lamp manufactured by Bausch and Lomb, type 3183110 at a distance from the lamp to sample of 15 inches and 110 voltsoperating the light. By various supply voltage settings, the ersistivitywas varied over the range indicated. The results of varying the loadresistance established at 10,

20, and 40 microamps is contained in Table VI following:

TABLE VI Load Resistance, Load Current,

ohms amps It should be appreciated that even though temperature affectsthe resistivity of the gallium arsenide photoresistor 1, it isunnecessary to provide a compensation in the current controlling circuitfor this phenomenon inasmuch as any reason for load resistance change oran effective total change in resistivity including the gallium arsenidephotoresistor would merely tend to change the current through the loadwhich would be detected as an error signal and fed to the galliumarsenide photoresistor as an increase or decrease in light intensitythus compensating the resistivity of the controlled gallium arsenidephotoresistor 1 providing further control to maintain a constantcurrent. Such results obviously can be understood by studying the datawhich was conducted with no particular attempt at controlling thetemperature.

One of the more important uses for the current controlling galliumarsenide photoresistor and thermistor device in circuits is to make Halletfect and resistivity measurements on materials which have extremelyhigh resistivity at room temperature and below, and whose resistivitydecreases rapidly as the temperature is increased. An example of thetype material for which resistivity and Hall effect measurements aredesired is gallium arsenide which according to the data and the tablespresented in the specification herein varies in resistivity from as muchas 200 megohms at room temperature to 20,000 ohms at 225 C. It will beappreciated that, first of all, it will be necessary to control thecurrent through a sample during the measurements for Hall effect andresistivity as the temperature is being varied. Furthermore, a ratherhigh voltage will be required to obtain a reasonable sample current atlower temperatures. It is desirable to have a sample current at lowertemperatures. It is desirable to have a sample current of at least 10"amperes for the measurements, therefore, a voltage source of at least2,000 volts is indicated. This is one feature of the gallium arsenidephotoresistor, that it has the ability to withstand extremely highvoltages without breakdown.

It should be appreciated that many modifications and changes will becomereadily apparent to those skilled in the art from the reachingscontained herein, and such changes and modifications are deemed to bewithin the scope of the present invention which is limited only by theappended claims.

What is claimed is:

1. In the process of making intrinsic-appearing gallium arsenidecrystalline material by heating gallium in the presence of arsenicvapor, the step of doping said gallium arsenide with oxygen derived fromAS203.

2. A method of making intrinsic-appearing gallium arsenide comprisingthe steps of:

combining substantially pure gallium with an excess of substantiallypure arsenic to produce non-stoichiometric gallium arsenide;

and float zone refining said gallium arsenide in the presence of arsenicvapor until the resistivity of said gallium arsenide increases toseveral rnegohm-cm.

References Cited Intermetallic Crystals Grown by New Zone RefiningTechnique, Bell Laboratories Record, vol. 36 (July 1958) p. 267.

Weisbcrg et al., Materials Research on GaAs, etc. Properties of Bimetal& Compound Semiconductors, ed.

by H. C. Gatos, Interscience Publ. (1960) pps. 4855. TK 7872 S4 C5.

LEON D. ROSODOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

1. IN THE PROCESS OF MAKING INTRINSIC-APPEARING GALLIUM ARSENIDECRYSTALLINE MATERIAL BY HEATING GALLIUM IN THE PRESENCE OF ARSENICVAPOR, THE STEP OF DOPING SAID GALLIUM ARSENIDE WITH OXYGEN DERIVED FROMAS2O3.